1. Field of the Invention
The present invention relates to semiconductor optical modulation devices, Mach-Zehnder interferometer type semiconductor optical modulators, and methods for producing semiconductor optical modulation devices.
2. Description of the Related Art
PCT International Publication No. WO 2004/081638 (Patent Document 1) discloses a semiconductor optical modulator. In this semiconductor optical modulator, the refractive index of a semiconductor layer is changed by applying an electric field to the semiconductor layer using the quantum-confined Stark effect (QCSE) or the Pockels effect. Therefore, optical light is modulated by changing the refractive index of the semiconductor layer according to changing the electric field. Japanese Unexamined Patent Application Publication No. 2008-66318 (Patent Document 2) discloses a semiconductor tunable laser. In this semiconductor tunable laser, the refractive index of a semiconductor layer is changed by injecting a current thereinto. A modulated optical signal is emitted form the semiconductor tunable laser by changing the injection current.
In this semiconductor tunable laser disclosed in Patent Document 2, the refractive index of the semiconductor layer is changed by injecting a stationary current or carriers to, for example, an optical waveguide layer. For the changed refractive index to be reset, carriers need to disappear. The disappearance of carriers, however, is determined by the carrier lifetime, which requires a long period of time. Accordingly, an optical modulator operating by changing an injection current has a much lower response speed than that operating by changing an electric field.